So what is the prime optimal condition for the Earth? Are there settings, i.e. temperature and humidity, ideal for life on the planet? That is the basic worry, isn't it, that life ceases to exist? Is it in our best interest to have a miles thick ice sheet scraping our homes off of the surface? How do we weigh the factors and come up with Utopia? Is there enough data to reach a conclusion or even a consensus?
Obviously there is no standard. Do you mean human life or life in general? If you mean life in general, look at areas that haven't been affected by humans (if there are any left).
When it comes to the Earth, the only "normal" is "current." The Earth's climate fluctuates around various cycles, including annually, El Nino, 11-yr sunspot cycles, 1500-year climate cycles, Milankovich cycles... Add to that the fact we've been cooling for a few billion years...
